1. Field of the Invention
This invention relates to an organic fiber cord for rubber reinforcement capable of largely enhancing durability and dimensional stability of rubber articles without exerting upon various properties of the rubber article as well as a pneumatic radial tire using the same as a carcass ply.
2. Description of Related Art
At the present time, the organic fiber cords for rubber reinforcement, particularly fiber cords treated with an adhesive are widely used as a reinforcement for rubber articles such as tires, conveyor belt, V-belt, hoses and the like. As a material of the fiber cord, there are preferably used organic fibers such as rayon fiber, polyamide fiber, polyester fiber, aramide fiber and so on. In order to enhance the performance of a final rubber article, there are used a method of improving properties of rubber material, a method of changing a structure or form of a rubber article and the like, and also it is very important to improve the properties of the fiber cord used for the reinforcement.
Recently, polyester fibers become mainly used as a material for a fiber cord of a carcass ply in a pneumatic radial tire for passenger cars because they are high in the strength and dimensional stability as compared with the conventional rayon fiber and nylon fiber. Furthermore, polyester fibers having a dimensional stability higher than that of the conventional polyester fiber or so-called high modulus and low heat-shrinkable polyester fibers are produced with the advance of high-speed and high-stress spinning technique, which are used as a fiber cord for the carcass ply to attempt the improvement of steering stability and uniformity of radial tires.
Especially, the heat shrinkability of the carcass ply cord is closely related to the deformation of the tire at a high temperature (170-180.degree. C.) applied in the tire building (vulcanization). It is known that as the heat shrinkage factor becomes small, it tends to increase the roundness of the tire and improve the tire uniformity (e.g. JP-A-5-71033, JP-A-6-179304, JP-A-3-167002, JP-A-5-16606, JP-A-6-24208 and the like).
Since the polyester fiber cords mainly used in the carcass ply of the radial tire for the passenger car are high in the strength and dimensional stability as mentioned above, they are effective to improve the steering stability and uniformity of the radial tire. As shown in FIG. 1, however, the radial tire has a general structure that a carcass ply 14 is wound around around each of bead cores 12A, 12B from inside toward outside to form a turnup portion and an end 20A, 20B of the turnup portion is at a freely moving state, so that there are caused the following problems. That is, when the heat shrinkage factor of the cord used in the carcass ply is simply decreased, the self-shrinkage factor of the cord in the turnup portion becomes small, so that the disorder is caused in the arrangement of the cords at the end of the turnup portion in the carcass ply and hence the fatigue resistance of the turnup portion is degraded to largely lower the durability of the tire. On the other hand, when the dry heat shrinkage factor of the cord is simply large, the dimensional stability is degraded to reduce or deform the resulting product. Consequently the roundness is lost and the uniformity is considerably degraded in case of the products attaching importance to the dimensional stability such as tires or the like. In any case, it is difficult to simultaneously establish dimensional stability and durability of the rubber article reinforced with the organic fiber cords only by using the conventional fiber cords for rubber reinforcement treated with the adhesive.
In tires, particularly passenger car tire among the rubber articles, steering stability and ride comfortability are very important among various running performances. The steering stability considerably exerts upon the running performance of the vehicle and becomes occasionally an important property concerning the safeness of the vehicle, while it is needless to say that the ride comfortability is an important property exerting upon the pleasantness of the passengers. In general, when the rigidity of the sidewall portion of the tire is enhanced for the improvement of the steering stability, the ride comfortability is rather lowered. For this end, various research and development for the improvement of these properties are actively carried out watching the carcass ply or cap layer of the tire, but it is said that there is a limit in the simultaneous improvement of these properties because the steering stability and the ride comfortability are so-called conflicting requirements in the construction of the tire. In this connection, it has first been made possible to simultaneously establish the steering stability and the ride comfortability in the same tire by developing a tire having two or more inflection points on its carcass profile line in which centers of curvature of arc are existent outside and inside the carcass profile line as described in U.S. Pat. Nos. 5,238,039 and 5,370,169.
In the tire having centers of curvature located outside and inside the carcass line and two or more inflection points as described in U.S. Pat. Nos. 5,238,039 and 5,370,169, when simple low-shrinkage cords are applied to the carcass ply, the carcass line aimed by the tire designer is obtained in the carcass ply extending between the bead cores, but the properties are disordered in the turnup portion of the carcass ply wound around the bead core toward outside due to the small self-shrinkage of the cord and hence the tire durability may largely be degraded. On the other hand, when simple high-shrinkage cords are applied to the carcass ply, the properties of the cord are sufficiently maintained in the turnup portion, but it is difficult to render the carcass ply extending between the bead cores into the carcass line aimed by the tire designer.